1. The Field of the Invention
This invention relates to the recovery of fresh or potable water from a high salt content feed solution, such as sea water, by reverse osmosis.
2. The Prior Art
Reverse osmosis has emerged from its previous status as a scientific curiosity to a practical reality. Its commercial use, however, has thus far been generally in equipment for purifying brackish and relatively low solute content waters. Problems arise in purifying sea water because of the high pressures required, the corrosive nature of sea water, and the need for a very low by-pass type membrane assembly. High pressure results in compaction of the membrane, which lowers the membrane water flux performance, and requires use of expensive, high-strength components which, even with technically successful units, has resulted in high capital costs.
Two general approaches have been proposed or tried for conversion of sea water by reverse osmosis. One approach has been to attempt to convert sea water in one stage directly to fresh or potable water. This has required high pressures (of the order of 800 to 1,200 psi or more) and very tight membranes to provide the salt rejection required, i.e., over 99%, in order to reduce the nominal 35,000 ppm TDS (total dissolved solids) content in the original sea water to, for example, below 1,000 or 500 ppm, which is acceptable as potable water. Purified water flux through such tight membranes is lowered, hence, membrane life is sharply curtailed by compaction caused by the extremely high pressure. An advantage of this approach is that, under the right conditions, a single stage sea water conversion unit might be built at less capital cost than a two stage plant which requires essentially duplicate equipment and generally a somewhat greater amount of total membrane area, since the feed water must pass through two membranes before reaching the desired product quality. This is somewhat offset by designing for higher unit water fluxes in the two stage units.
Multiple stage plants heretofore employed or proposed have employed tight membranes (rejection of 95% or higher) and high pressure (of the order of 800 psi to 1,200 psi) in a first stage to produce an intermediate product water containing in the order of 2,500 to 5,000 ppm TDS, which can be readily treated in a second stage as a type of brackish water. The first stage product flux under these conditions has been greater than in the single stage design, and the intermediate product water can be treated by commercially available reverse osmosis units and membranes used for treating brackish water. This type of two stage process, however, still suffers from the high pressure, expensive construction material, and membrane life problems of the single stage design; and additionally must bear the additional capital cost of the second stage equipment.
Typical of such operation is the process described in U.S. Pat. No. 3,457,170, in which a pressure of 800 psi is employed in the first stage and a pressure of 500 psi is employed in the second stage. Quite distinct from this, Applicant employs low pressures within similar ranges for both stages, rather than the high pressure first stage and lower pressure following stage, and eliminates the need for any operation above about 550 psi. Moreover, Applicant employs a loose type of membrane in his first stage and a tight membrane in his second stage, which is counter to prior art teachings that tight membranes and high pressures should be employed for treatment of high salt content solutions.
The process of this invention solves problems encountered in prior art sea water conversion processes and makes possible use of low pressures and inexpensive materials in a multiple stage process to realize extended membrane life, improved production rates, and low cost.